This proposal consists of four separate but interrelated projects directed at understanding the mechanisms of neurotransmitter release and variable synaptic efficacy. The objectives of the proposed research are (1) to determine whether the quantum of transmitter release is composed of about seven subunits as proposed by Kriebel et al. (1976), (2) to determine whether d-tubocurarine has a presynaptic effect on transmitter release at the neuromuscular junction, (3) to develop a quantitative working hypothesis to account for stimulation-induced changes in transmitter release under conditions of normal quantal contents, and (4) to determine whether endplate potentials, miniature endplate potentials, and subminiature endplate potentials arise from similar or different transmitter release mechanisms. Depending on the experiment, estimates of stimulation-induced changes in transmitter release at the vertebrate neuromuscular junction (frog, mouse, and rat) will be made from measurements of endplate potentials, miniature endplate potentials, and subminiature endplate potentials recorded intracellularly or from voltage clamp measurements of the endplate currents that give rise to these potentials. Iontophoretically-induced endplate currents and noise analysis will be used to determine whether the experimental procedures affect postsynaptic sensitivity and the operation of the acetylcholine receptor. Models for the mechanism of transmitter release will be formulated and evaluated by comparing experimental results with predicted results obtained by simulation of the proposed mechanisms of release on a digital computer.